(a) Technical Field
The present invention relates to compositions and methods for manufacturing a cathode for a secondary battery. More particularly, the present invention relates to compositions and methods for manufacturing a cathode for a secondary battery, where the cathode includes manganese fluorophosphate with lithium or sodium.
(b) Background Art
As the use of portable small-sized electronic devices has become widespread, there has been an active interest in developing new types of secondary batteries such as, for example, nickel metal hydrogen or lithium secondary batteries. For example, a lithium secondary battery uses carbon (such as, e.g., graphite) as an anode composition, lithium-containing oxide as a cathode composition, and a non-aqueous solvent as an electrolyte. Lithium is a metal that has a very high tendency to undergo ionization; consequently, lithium can achieve a high voltage. Thus, lithium is often used in the development of batteries having high energy density.
In a lithium battery, a cathode composition typically includes a lithium transition metal oxide containing lithium in which 90% or greater of the lithium transition metal oxide includes layered lithium transition metal oxides (such as, e.g., cobalt-based, nickel-based, cobalt/nickel/manganese ternary-based, and the like). However, when such layered lithium transition metal oxides are used as a cathode composition, lattice oxygen within the layered lithium transition metal oxides may become deintercalated and participate in an undesired reaction under non-ideal conditions (such as, e.g., overcharge and high temperature), thereby causing the battery to catch fire or explode.
In order to overcome the disadvantages of such layered lithium transition metal oxides, researchers have considered cathode compositions having a spinel or olivine structure. In particular, it has been suggested that a cathode composition including a spinel-based lithium manganese oxide having a three dimensional lithium movement path, or a polyanion-based lithium metal phosphate having an olivine structure, instead of a layered lithium transition metal oxide, may prevent problems in lithium secondary batteries that arise from decreased stability in layered lithium transition metal oxides as a result of cathode deterioration.
The use of the spinel-based lithium manganese oxide as a cathode material has been limited because repeated cycles of battery charging and discharging results in lithium elution. Moreover, spinel-based lithium manganese oxide containing compositions display structural instability as a result of the Jahn-Teller distortion effect.
The use of olivine-based lithium metal phosphates, such as iron (Fe)-based phosphate and manganese (Mn)-based phosphate, as a cathode material has also been limited because these compounds have low electrical conductivity. However, through the use of nano-sized particles and carbon coating, the problem of low electrical conductivity has been improved, and thus the use of olivine-based lithium metal phosphates as a cathode composition has become possible.
For example, it has been recently reported that fluorophosphates may be useful as a cathode material. The fluorophosphate has the following formula: A2MPO4F, where A represents Li or Na, and M represents a transition metal such as Mn, Fe, Co, Ni, V, or a mixture thereof. Theoretically, the fluorophosphate of formula A2MPO4F is expected to have a capacity about twice as high as a conventional lithium metal phosphate since it has two Na atoms. For example, in the case where a fluorophosphate having the formula Na2MPO4F (M=Mn, Fe, Co, Ni, V or a mixture thereof) is used as a cathode material for a lithium secondary battery, sodium is deintercalated during the initial charging step and lithium is intercalated during the initial discharging step. In the following cycles of battery charging and discharging, alternating intercalation and deintercalation of lithium occurs during the charging and discharging process. Similarly, in the case where Na2MPO4F (M=Mn, Fe, Co, Ni, V or a mixture thereof) is used as a cathode material for a sodium battery, the intercalation and deintercalation of sodium is carried out during charging and discharging.
U.S. Pat. No. 6,872,492 discloses an example of using a fluorophosphate including sodium, such as NaVPO4F, Na2FePO4F, or (Na,Li)2FePO4F, as a cathode material for a sodium based battery. However, the example is limited to a sodium based battery, and has not been attempted for a lithium battery.
As another example of the conventional art, sodium iron fluorophosphate (Na2FePO4F) has been used as a cathode material for a lithium secondary battery, and the structure of Na2FePO4F and its electrochemical characteristics have been disclosed. However, iron-based Na2FePO4F suffers from a major disadvantage as a cathode material because it has a low charge/discharge potential (about 3.5 V), which is similar to an iron-based olivine material. Attempts to overcome this disadvantage of Na2FePO4F have been made by using manganese-based Na2MnPO4F, which has a higher potential (4V) compared to iron-based Na2FePO4F. Unfortunately, Na2MnPO4F also suffers from a major disadvantage as a cathode material because of electrochemical inactivity due to the low electrical conductivity of a polyanion-based material.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention.